1. Field of the Invention
One embodiment of the present invention relates to a semiconductor device. In this specification, a semiconductor device refers to a semiconductor element itself or a device including a semiconductor element. As an example of such a semiconductor element, for example, a thin film transistor can be given. Therefore, the semiconductor device includes a liquid crystal display device, a memory device, and the like.
2. Description of the Related Art
A semiconductor device incorporates a variety of circuits for desired operations. One example is an oscillator circuit capable of producing a continuous alternating current. One type of oscillator circuit is a ring oscillator (see, for example, Patent Document 1). Another example, other than the oscillator circuit, is a delay circuit. One type of delay circuit is an inverter chain (see, for example, Patent Document 2).
As a circuit incorporated in a semiconductor device, a complementary metal oxide semiconductor (CMOS) circuit which is a combination of p-channel and n-channel transistors is widely used. However, a p-channel transistor and an n-channel transistor greatly differ in field effect mobility, leading to a variety of inconveniences. The combination of p-channel and n-channel transistors also has the problem of process complication when manufactured over the same substrate.
In view of that, there have been attempts to configure a variety of logic circuits only with transistors having the same conductivity type. For the transistors having the same conductivity type, control of threshold voltage with back gates is particularly desired (for example, see Non-Patent Document 1).